CREATE OR REPLACE FUNCTION getvalue (
par int
)
RETURNS mytype AS
$body$
DECLAREretval mytype;
begin
...
return retval;
end;
body$
LANGUAGE 'plpgsql'

update t set f=getvalue(11) where id = 1

update t set f=getvalue(11)::mytype where id = 1

CREATE TABLE public.t(
  id INTEGER NOT NULL,
  value public.mytype
)

-- 1
CREATE TYPE sqlvariant AS (
  i INTEGER,
  g UUID,
  bi BIGINT,
  x TEXT,
  bl BOOLEAN
);

-- 2
create table t
(
id serial primary KEY,
val sqlvariant);

-- 3
CREATE OR REPLACE FUNCTION getval (
  par anyelement
)
RETURNS text AS
$body$
declare retvalue sqlvariant;
begin
IF par IS NULL THEN RETURN (NULL,NULL,NULL,NULL,null)::sqlvariant; END IF;
IF pg_typeof(par)::text='integer'  	THEN RETURN (par::integer,null,NULL,NULL,NULL)::sqlvariant; END IF;
IF pg_typeof(par)::text='uuid'  	THEN RETURN (null,par::uuid,NULL,NULL,NULL)::sqlvariant; END IF;
IF pg_typeof(par)::text='bigint'  	THEN RETURN (null,null,par::bigint,NULL,NULL)::sqlvariant; END IF;
IF pg_typeof(par)::text='text'  	THEN RETURN (null,null,null,par::text,NULL)::sqlvariant; END IF;
IF pg_typeof(par)::text='boolean'  	THEN RETURN (null,null,null,null,par::boolean)::sqlvariant; END IF;
RETURN NULL;
end
$body$
LANGUAGE 'plpgsql';

-- 4
insert into t(val) select getval(1::int); -- error
insert into t(val) select getval(1::int)::sqlvariant; -- ok